High-critical-current-ratio superconducting joint between Ba0.6K0.4Fe2As2 tapes fabricated by angle-polishing method

Superconducting joints of Ba0.6K0.4Fe2As2 (BaK-122) tapes were fabricated by a simple cold-pressing method. The interfaces of the superconducting cores were prepared by two different methods; (i) peeling off a part of the Ag sheath (PO joint) and (ii) angle polishing of one edge of each tape (AP joint). For the PO joint, a critical current ratio (CCR =Icjoint/Ictape, where Ictape and Icjoint are critical currents in tapes and joint parts, respectively) of 30% at 4.2 K and 3.5 T was obtained. Scanning electron microscopy revealed that the jointing area was not fully connected when the applied pressure was insufficient, and that cracks were induced in the superconducting core around the ends of the joint when the pressure was too high. These are considered to be factors that caused lowering of a CCR value. On the other hand, in the case of the AP joint, the formation of such gaps in the joint interface as well as inhomogeneous deformations and cracking around the joint-ends were successfully avoided. As a result, high CCR values of 80 ∼ 90% at 4.2 K under 0.5 ∼ 3.5 T were achieved, demonstrating that superconducting wires/tapes with sufficient length can be fabricated based on iron-based superconductors, which possibly opens up the potential for high-field magnet application.